Process to eliminate air pollutants which result from the combustion of fuels containing sulphur

ABSTRACT

Process to eliminate air pollutants which result from the combustion of fuels containing sulphur, permitting the transformation of substantially all the existing SO 2  and SO 3  produced by the combustion of fuels to ammonium sulphate.

TECHNICAL FIELD

The present invention refers to a process aimed at the elimination ofair pollution caused by the burning of Sulphur containing fuels in allkinds of industry and equipment of all sizes. The process requires lessinvestment than other alternative processes and produces a commercialend-product.

BACKGROUND ART

One of today's main concerns has been to eliminate air polluting agentsresulting from the burning of Sulphur containing fuels, withoutgenerating inadequate by-products. Many of the processes presently usedrequire large investments in equipments and installations, normallywithout return or with high added operational costs.

DISCLOSURE OF INVENTION

Aiming at solving or minimizing these problems, this process has beendevelopped. Basicaly simple, easily retrofitted into the pollutingequipments, with low investment costs, this process produces a highvalue end-product. Specifically, the aim of this invention is totransform all Sulphur Dioxide (SO₂) and Sulphur Trioxide (SO₃) existingin the combustion gases, to Ammonium Sulphate. This is obtained throughthe reaction of SO₃ and SO₂ with Ammonia, preferably in gaseous state,which is injected directly into the combustion gases. The collectedstable end-product is Ammonium Sulphate [(NH₄)₂ SO₄ ] which is separatedfrom the flue gases.

These and other objectives of this invention are obtained by thisprocess which comprises the following steps: undertake at least oneinjection of Ammonia (NH₃) in the combustion gases at temperaturesbetween 250° C. and 600° C., in quantity sufficient to react with allthe SO₃ which is present and with part of the existing SO₂ ; cool thegases to a temperature lower than about 65° C.; separate the submicronand micron size particles from the combustion gases; collect the (NH₄)₂SO₄ from the separating equipment, and send the desulphurized combustiongases to the flue.

Considering the possibility of providing more then one injection pointof Ammonia (NH₃), this process has foreseer the division of theinjection into: a first injection of gaseous NH₃ (Ammonia) into the 250°C. to 600° C. combustion gases and in a quantity sufficient to reactwith all the present SO₃, and a second injection of gaseous NH₃ at aregion where the combustion gases are at a 65° C. to 250° C. temperaturerange and in a quantity sufficient to react with at least a part of theexisting SO₂.

In this case, the first injection is directed mainly the neutralizationof the SO₃.

In case of injection of all the Ammonia (NH₃) in the 250° C. to 600° C.combustion gases temperature range, the NH₃ will react with all presentSO₃ and the rest of NH₃ will intermix with the combustion gases stream,up to the point where the temperature permits the occurence of anirreversible reaction between the SO₂ and NH₃.

As already established technically, the SO₃ is a gas formed during thecombustion of Sulphur containing fuels, by the reaction of gaseous SO₂with molecular Oxygen (O₂), by the reaction of SO₂ in the flame withatomic Oxygen and by catalytic oxidation of SO₂ in the heat transfersurfaces under high temperatures. The corresponding reactions are thefollowing:

    S+O.sub.2 →SO.sub.2

    2SO.sub.2 +O.sub.2 →2SO.sub.3

    S+3(O)→SO.sub.3

Without the presence of catalysers, the burning of Sulphur containingfuels results in up to 5% SO₃ in the total of SO_(x) compounds. In spiteof the fact that gaseous Ammonia reacts with SO₃ in the temperaturerange of 250° C. to about 600° C., it has been verified that the idealtemperature range is 300° C. to 400° C. In this range, the reation ofSO₃ with NH₃ is instantaneous resulting in (NH₄)₂ SO₄ (Ammonia Sulphate)and eliminating the possibility of corrosion in the surfaces which wouldhave been contacted by the SO₃. This corrosion occurs due to theformation of liquid Sulphuric Acid (H₂ SO₄) when SO₃ and water ofcombustion (H₂ O) combine at temperatures below the Sulphuric Acid dewpoint. The quantity of injected NH₃ is sufficient to insure theformation of (NH₄)₂ SO₄ (Ammonium Sulphate).

Smaller quantities of NH₃ would result in the formation of HydrogenAmmonium Sulphate (NH₄ HSO₄), which is a corrosive compound. Thecorresponding reactions are the following:

    2NH.sub.3 +H.sub.2 O+SO.sub.3 →(NH.sub.4).sub.2 SO.sub.4

    NH.sub.3 +H.sub.2 O+SO.sub.3 →NH.sub.4 HSO.sub.4

The minimum calculated quantity of NH₃ to avoid formation of NH₄ HSO₄ in3% Sulphur bearing fuel oils, for example, is 0.08% of the oil weight.For higher levels of Sulphur, the percentage will correspondingly higher(up to 9%). The reaction product (NH₄)₂ SO₄ is stable and is carriedtogether with the combustion gases' stream. As previously mentioned,this process is directed towards a complete and reliable elimination ofthe SO₂ resulting from the combustion of liquid, solid or gaseous fuels.

To neutralize the SO₂, the gaseous NH₃ is injected in one or morelocations of the equipment, where the combustion gases are at a range of65° C. to 600° C. If the gaseous NH₃ is totally injected in a 200° C. to600° C. region, the excess portion which does not participate in thereaction with SO₃, is carried in the combustion gases' stream, formingan intimate mixture which will react when the right temperature range isreached.

At 120° C. the quantities of SO₂ which react with NH₃ increases, and asthe temperature of the gases decreases to about 108° C., an equilibriumsituation is reached with 35% to 40% of the existing SO₂ having reactedwith NH₃ according to the following reversible reations:

    NH.sub.3 +H.sub.2 O+SO.sub.2 ⃡(NH.sub.4)HSO.sub.3 Ammonium Hydrogen Sulphite

    (NH.sub.4)HSO.sub.3 +NH.sub.3 ⃡(NH.sub.4).sub.2 SO.sub.3 Ammonium Sulphite

    (NH.sub.4).sub.2 SO.sub.3 ⃡2NH.sub.3 +H.sub.2 O+SO.sub.2

It has been experimentaly verified that this equilibrium situation ismaintained until temperatures in the range of 65° C. are attained.

Between 65° C. and 48° C. it has been experimentaly proven that thisreaction is stable with the simultaneous oxidation of Ammonium Sulphiteto Ammonium Sulphate according to the following reactions:

    2NH.sub.3 +H.sub.2 O+SO.sub.2 →(NH.sub.4)2SO.sub.3

    2(NH.sub.4).sub.2 SO.sub.3 +O.sub.2 →2(NH.sub.4).sub.2 SO.sub.4

This oxidation is processed by existing combustion air in excess or byan extra air antrainment into the gases, just before the separationsystem. The (NH₄)₂ SO₄ (Ammonium Sulphate) is a stable, solid, submicronto a low value micra sized, product.

The above mentionned facts lead to verified results, if the combustiongases are released at higher temperatures then 65° C., part of the SO₂(even if already reacted) might be released to the atmosphere, due tothe reversible nature of the reactions at that higher range.

The separation of the stable end-product (NH₄)₂ SO₄ (Ammonium Sulphate)from the combustion gases' stream, can be accomplished by dry methodssuch as Electrostatic Precipitators or other kinds of filters, or moreeconomically by wet processes such as water absorbers.

The absorption of (NH₄)₂ SO₄ (Ammonium Sulphate) in water is possibledue to the highly soluble nature of this product.

DESCRIPTION OF THE DRAWINGS

The process will now be briefly outlined so as to show an example of afeasible and tested lay-out of the Desulphurization of Combustion Gaseswith Ammonia.

FIG. no. 1 represents a flow diagram of the main components of aninstallation comprising the equipment where the Sulphur bearing fuel isburned, two distinct Ammonia injection points, the (NH₄)₂ SO₄ wetabsorption system, an entrained water separator, the desulphurized fluegases released to atmosphere and the concentrated Ammonium Sulphatesolution collection system.

FIG. no. 2 illustrates one of the several assemblies of the Ammoniainjection elements, in adequate locations of the combustion gases'stream.

FIG. no. 3 shows a typical configuration of the NH₃ injection tubes.

BEST MODE OF CARRYING OUT THE INVENTION

As shown in FIG. 1, this process is applied in industrial installationssuch as boilers, furnaces, ovens, dryers, etc (1), burning Sulphurbearing fuels. Heat recovery equipment (2) may or not be present. In thepresent process, NH₃ coming from a storage tank (3) is distributed ingaseous or liquid form. If as a liquid, NH₃ is vaporised or not andinjected initialy in a region where the gases are preferably between250° C. and 600° C. and before the heat recovery equipment. The quantityof the first injection is, as previously explained, adequate to insurecomplete reaction with the SO₃, therefore eliminating corrosion problemsin the heat recovery or other metallic or non metallic components of theexisting installation. This permits the use of cheaper constructionmaterials or results in longer life of the existing ones.

The combustion gases which leave the heat recovery equipment (2) in theexample shown, are conducted by means of a duct (5) and ventilator (4)to a point where a second injection of gaseous NH₃ is undertaken atlower temperature levels.

Even if the gaseous NH₃ is injected at only one location, the quantitywill always be such as to react initialy with the existing SO₃ andfurther on at lower temperatures with the SO₂.

The combustion gases, after the last NH₃ injection, are sent along theduct (5) and into an appropriately dimensioned duct (6). This duct (6)has a battery of atomizing nozzles (6a) in adequate number and sizing.

These nozzles spray a recirculating solution of Ammonium Sulphate intothe combustion gases. This solution is recirculated under pressure by apump and piping (7).

A water make-up system (7a) is hooked up into this r recirculatingsystem. This pre-washing phase insures initial absorption of theexisting (NH₄)₂ SO₄ particles and a severe temperature drop of thecombustion gases. Depending on the inlet temperature of the gases tothis pre-washer/absorber, the completion of the SO₂ reaction to (NH₄)₂SO₄, may also happen in this region.

In the second stage of the (NH₄)₂ SO₄ (Ammonium Sulphate) absorption inwater, the combustion gases pass through a high pressure injector (8a)(or additional nozzles) spraying (NH₄)₂ SO₄ solution into the gaseousstream, which is assembled on top of a solution collection tank (8). Atthis phase the simultaneous retention of the soluble Ammonium Sulphateparticles and of the insoluble particles such as carbon, inorganicmaterials, etc, is achieved.

The concentrated solution of Ammonium Sulphate may be continuously orperiodicaly taken away from the collection tank (8) into a storage tank(9).

This volume is substituted by make-up water. One of the advantages ofthis wet separation process is that the insoluble and undesirableparticles can be easily separated from the dissolved Ammonium Sulphatesolution by conventional filters or gravity settling.

The system has also foreseen, for the cases where water entrained asmist should not be lost, the installation of a low pressure dropdemister (10) which recovers part of the water which is returned to thesolution tank (8) by piping (11).

The combustion gases leave the solution tank (8) totaly desulphurizedor, if desired, within the SO₂ emission levels specified by the localEnvironmental Control Agency. The emission level is easily adjusted bythe variation of the Ammonia flow to be injected, with the use ofproportioning or manual valves and flowmeters.

The complete system can also receive a pH controller of the tank (8)solution to insure continuous neutralization of the SO₂.

Direct or indirect measurement systems of the Ammonium Sulphateconcentration in the solution contained in the tank (8) can also beadded.

The duct (5) which receives the combustion gases, can also remainconnected to the existing chimney (12) through a normaly closed gatevalve which functions as a by-pass to the separation system.

As shown in FIGS. 2 and 3, the Ammonia gas injection is made in the gasducts through perfurated tubes (20), the number and disposition of thetubes being determined by the calculated Ammonia gas flow, and by theobjective of obtaining complete intermixing with the combustion gases.

The tubes are preferentialy built out of mild steel and completely crossthe combustion gas ducts, each tube being divided in three equal parts,with different number and sizes of holes (21), as better detailed inFIG. 3. The first part has two series of holes (21) with a diameter dand a spacing of x. The second part has two lines of d₁ diameter holes,where d₁ is larger than d and spaced in an interval x₁, smaller than x.Likewise the third part has d₂ diameter holes (d₂ larger than d₁) spacedat x₂ intervals smaller than x₁.

The usual installation requires two parallel tubes (FIG. 2). In thiscase, the NH₃ gas will enter one tube from the right end of the ductwhile the other tube enters from the left.

The NH₃ gas always enters the tube going through the d diameter, xspaced holes (21).

A plug (30) blocks the end of each tube. The two series of holes of eachtube (FIG. 3) are located so that a hole of one series is always inbetween two holes of the other series.

We claim:
 1. A process for eliminating air pollutants in the form ofsulfur dioxide and sulfur trioxide resulting from the combustion ofsulphur bearing fuels and for producing a liquid consisting essentiallyof ammonium sulphate as a useful commercial end-product, which comprisesthe following steps:providing at least one injection of Ammonia intocombustion gases which are at a temperature of 250° C. to 600° C., theAmmonia being injected in sufficient quantity to transform, intoAmmonium Sulphate, substantially all the existing SO₃ and SO₂ producedby the combustion of these fuels, a portion of the injected Ammoniareacting with substantially all of said SO₃ to produce AmmoniumSulphate, the remainder of said Ammonia intermixing with said combustiongases; cooling the combustion gases having intermixed Ammonia to atemperature lower than 65° C. in the presence of air, whereby at least aportion of the remaining Ammonia reacts with substantially all of saidSO₂ to produce ammonium sulfite which is simultaneously oxidized toproduce Ammonium Sulphate; passing these gases through a separationsystem adequate for submicron to low value micra particulates;collecting the Ammonium Sulphate end-product from the separation system;and releasing the desulphurized gases to the atmosphere.
 2. A process asclaimed in claim 1, characterized in that the Ammonia injection is madewith the combustion gases at a temperature range of about 250° C. to400° C.
 3. A process for eliminating air pollutants in the form ofsulfur dioxide and sulfur trioxide resulting from the combustion ofsulphur bearing fuels and for producing a liquid consisting essentiallyof ammonium sulphate as a useful commercial end-product, which comprisesthe following steps:providing a first injection of Ammonia into thecombustion gases when these are at a 250° C. to 600° C. temperaturerange and in sufficient quantity to react with all SO₃, formed by thecombustion of these fuels; providing a second injection of Ammonia intothe combustion gases when these are at a 65° C. to 250° C. temperaturerange, and in a sufficient quantity to react with at least a sufficientpart of the SO₂, so that the SO₂ remaining is within the local limitsfor emission of SO₂, as established by the Environmental Authorities;cooling the combustion gases to a temperature lower than 65° C. in thepresence of air, whereby at least a portion of the remaining Ammoniareacts with substantially all of said SO₂ to produce ammonium sulfitewhich is simultaneously oxidized to produce Ammonium Sulphate; passingthese gases through a separation system adequate for submicron to lowvalue micra particulates; collecting the Ammonium Sulphate end-productfrom the separation system; and releasing the desulphurized gases to theatmosphere.
 4. A process as claimed in claim 3, characterized in thatthe first injection of Ammonia is made with the combustion gases whenthey are at a temperature range of 300° C. to 400° C. and secondinjection of Ammonia with the gases at 65° C. to 120° C. range.
 5. Aprocess as claimed in any one of the claims 1 or 2, characterized inthat the separation means include absorbers with water, with theresulting Ammonium Sulphate Solution being separated from thedesulphurized combustion gases.
 6. A process as claimed in claim 5,characterized in that the cooling of the combustion gases totemperatures under 65° C. is also completed in the separation system. 7.A process as claimed in any one of the claims 1 to 4, characterized inthat the Ammonia is injected in gaseous state.
 8. A process as claimedin claim 7, characterized in that the injection of Ammonia gas is madeby means of perforated tubes installed transversely to the combustiongases' stream.